Difference Between HbA and HbF

The key difference between HbA and HbF is that HbA refers to adult hemoglobin which is an α2β2 tetramer while HbF refers to fetal hemoglobin, which is an α2γ2 tetramer that can bind to oxygen with greater affinity than HbA.

Hemoglobin is a complex protein molecule in red blood cells that carries oxygen from the lung to the body tissues and returns carbon dioxide from the body tissues to the lung for the removal. Iron is an essential element required for blood production, and it’s a constituent of hemoglobin. There are two main forms of hemoglobin as fetal hemoglobin (HbF) and adult hemoglobin (HbA). Here, the HbF is the primary oxygen transport protein in the human fetus, and adult hemoglobin replaces HbF approximately six months postnatal. Adult hemoglobin is the main form of hemoglobin present in human. Among HbF and HbA, HbF has a higher affinity for oxygen than HbA. Structurally, HbA is an α2β2 tetramer while HbF is an α2γ2 tetramer.

1. Overview and Key Difference
2. What is HbA
3. What is HbF
4. Similarities Between HbA and HbF
5. Side by Side Comparison – HbA vs HbF in Tabular Form
6. Summary

What is HbA?

HbA stands for adult hemoglobin, which is an α2β2 tetramer. It is an iron-containing red blood cell protein responsible for the transportation of oxygen from the lung to the body tissues and organs, and transportation of carbon dioxide from body tissues to the lung. It is a complex protein that consists of four small protein subunits and four heme groups bearing iron atoms. Hemoglobin has an affinity for oxygen. There are four oxygen binding sites located inside a hemoglobin molecule. Once hemoglobin becomes saturated with oxygen, the blood becomes bright red in colour. The second state of hemoglobin is known as deoxyhemoglobin as it lacks oxygen. At this state, the blood is dark red in colour.

Figure 01: HbA

Iron atom embedded inside the heme compound of hemoglobin mainly facilitates the oxygen and carbon dioxide transportation. Binding of oxygen molecules to Fe⁺² ions changes the conformation of the hemoglobin molecule. Moreover, iron atoms in the hemoglobin help to maintain the typical shape of the red blood cell. Therefore, iron is a vital element found in red blood cells.

What is HbF?

HbF stands for fetal hemoglobin which is the predominant form of hemoglobin in the fetus. HbF develops from the erythroid precursor cells. In fact, HbF appears in fetal blood after a few weeks of conception. HbF remains till six months of postnatal life as well. After that, adult hemoglobin replaces HbF completely. Similar to HbA, HbF is also a tetramer. But it contains two α-chains and two gamma subunits.

Figure 02: HbF

When compared to HbA, HbF has a higher affinity for oxygen. Therefore, the P50 of HbF is lower than the P50 of HbA. Due to the high affinity for oxygen, the oxygen dissociation curve of HbF is left-shifted compared to HbA. Further, this higher affinity of HbF for oxygen is important to obtain oxygen from the maternal circulation.

What are the Similarities Between HbA and HbF?

HbA and HbF are two isoforms of hemoglobin.
They are proteins that contain iron molecules.
And, they have an affinity for oxygen.
Also, both are tetramers that have four subunits.
Besides, both have same α-chains.

What is the Difference Between HbA and HbF?

HbA and HbF are two forms of hemoglobin. HbA is the adult hemoglobin, which is the main form of hemoglobin in humans, while the HbF is a predominant form of hemoglobin in the developing fetus. So, this is the key difference between HbA and HbF. Structurally, HbA has two alpha chains and two beta chains, while HbF has two alpha chains and two gamma chains. Therefore, this is also a significant difference between HbA and HbF. Moreover, HbF shows a higher affinity for oxygen than HbA.

The below infographic summarizes the difference between HbA and HbF.

Summary – HbA vs HbF

Hemoglobin is an iron-containing metalloprotein found in the red blood cells. It carries oxygen from the lungs to the body tissues and facilitates energy production. It also returns carbon dioxide from the body tissues to the lung for the removal from the body. HbF is the predominant form of hemoglobin in developing fetus while HbA is the main form of hemoglobin in human after six months postnatal. HbA is a tetramer composed of two alpha chains and two beta chains while HbF is a tetramer composed of two alpha and two gamma chains. Furthermore, HbF has a higher affinity for oxygen than HbA. So, this summarizes the difference between HbA and HbF.

Reference:

1. Kaufman, Daniel P. “Physiology, Fetal Hemoglobin.” StatPearls [Internet]., U.S. National Library of Medicine, 16 Apr. 2019, Available here.
2. “Fetal Hemoglobin.” Wikipedia, Wikimedia Foundation, 22 Sept. 2019, Available here.

Image Courtesy:

1. “The value of fugacity for a particular chemical species, i.e. a real gas is” By en:User:BerserkerBen – Uploaded by Habj (CC BY-SA 3.0) via Commons Wikimedia
2. “Hemoglobin F” By AngelHerraez – Own work – A rendering prepared by me, using Jmol, from 4MQJ at PDB (doi:10.2210/pdb4mqj/pdb) (Public Domain) via Commons Wikimedia